Can't set the channel for an outgoing webhook to `all_public_channels,all_private_groups,all_direct_messages`

Description:

When creating an outgoing webhook, it's not possible to set the channel to all_public_channels,all_private_groups,all_direct_messages.

Steps to reproduce:

1. Create an outgoing webhook
2. set the channel to all_public_channels,all_private_groups,all_direct_messages
3. save

Expected behavior:

Integration is saved

Actual behavior:

There's an error message in the UI saying "Invalid channel", and the integration is not saved

Server Setup Information:

• Version of Rocket.Chat Server: 6.2.1

Client Setup Information

Consistent across different browsers and OS

Additional context

Following https://forums.rocket.chat/t/outgoing-webhook-that-triggers-on-a-user-mention-or-direct-message/3488/2 and https://github.com/RocketChat/Rocket.Chat/issues/4662#issuecomment-331150973, I was assuming that this is the correct setting for listening on all messages in all channels.

Reproduce the outgoing webhook setup described in the issue using the channel value all_public_channels,all_private_groups,all_direct_messages, then trace the validation that produces the UI's "Invalid channel" error. Done means the integration saves successfully with that value and the relevant validation coverage passes.